But there are ways to spice things up, like planning to do something "illicit" with your partner.

This post is part of Relationships 101, a series which aims to help us all be happier and healthier in love — and to stop fighting over who should take out the trash.

If there's one "problem" relationship experts hear over and over again, it's this: The passion has faded. The routine has replaced the spontaneous.

Yet most of those experts will tell you this generally isn't a reason to freak out. If there is a problem, it's in how you're handling the boredom.

Over the past few months, I've asked sex and relationship therapists to share their top strategies for keeping the passion alive in a romantic relationship, and preventing ennui from creeping in. Here are the best tips I heard:

Accept that the waxing and waning of passion is normal

Couples therapist Rachel Sussman puts it bluntly. "Were we really put on this earth to have a monogamous sex life for 50 years and have passion the entire time for our partner?" she said when I interviewed her last year. "I don't think so."

So when couples come to see Sussman complaining about the lack of passion in their relationship, she wants them to know: This is normal.

People are worried "that something's wrong with them," she told me. They think "maybe something's wrong with the couple; maybe something's wrong with them individually."

Chances are, there's not. "People think, 'Oh, [passion] should just be there,'" Sussman said. "No! It shouldn't just be there. You have to create it."

One strategy Sussman recommends? Scheduling sex dates, right there on the calendar.

Plan to do something 'illicit' in your relationship

Tammy Nelson is a sex and relationship therapist, and the relationship expert at Ashley Madison, a website for people seeking affairs. Nelson told me the "fantasy of an affair" is simply that "you'll have that impulsive excitement."

But affairs come with risk, like potentially destroying your partner's trust in you and wrecking your own self-image.

So Nelson proposes that people aim to have that impulsive excitement within their own relationships. "You have to have an affair with your spouse," she said. Meet like strangers at a bar one night, for example.

As Nelson said, "You have to make something about your marital sex feel dangerous."